Not Applicable
Not Applicable
This invention relates to a fork-lift truck.
Fork-lift trucks prove to be universal-use equipment to pick up pallets from the floor and to handle and stack them. They may be provided with a combustion motor or an electric motor. The fork-lift truck picks up the load and carries it to the desired place of destination. The front wheels are supported on a wheel bearing means wherein this one, e.g. in fork-lift reach trucks, may also have wheel arms extending in the vehicle longitudinal direction between which the load can be lowered. Since the load is picked up outside the basis of standing a load moment will arise which might cause the fork-lift truck to tilt about the front axle. The load moment has to be compensated for by a corresponding righting moment. The purpose is served by the dead weight of the vehicle that can be increased by separate counterweights at the other end of the vehicle (counterweight-type fork-lift truck). Thus, a defined working load of the fork-lift truck is opposed by a relatively large dead weight which requires to be accelerated and decelerated again during each driving and braking action. Also, the dead weight will increase the resistance to vehicular motion as well as the loads imposed on the wheels and presupposes a sufficient load-carrying capacity of the floor. Therefore, if lifting heights are larger the ultimate load admissible requires to be reduced as otherwise the stability of the vehicle will be jeopardized, especially when the load is lifted.
Different attempts were made in the past to increase the stability in standing of a fork-lift truck. A mobile fork-lift truck having a rearwardly inclinable lift mast is known from the patent specification DE 1 047 719 open to public inspection. The document discusses several designs of fork-lift trucks. For improved stability, it describes that if a lift mast inclines backwards an automatic approach of the inclinable hinge of the lift mast and the front wheel to each other is effected simultaneously. The mentioned document also describes a lift mast hinged to the vehicle which may be swiveled by a hydraulic cylinder, this one being hinged to the lift mast and the vehicle. If the lift mast is inclined backwards the front wheels undergo forward swiveling at the same time. The known devices have numerous drawbacks when in practical use. When stacking goods the lift frame should stand as vertical as possible and, hence, the frame requires to be pivoted first from the inclined position to an upright one before a pallet can be picked up or delivered at larger heights. For the rest, it will be left to the driver, in the known case, to move to appropriate inclinations of the lift frame.
It is the object of the invention to provide a mobile fork-lift truck which, while having a dead weight as low as possible and being of a simple structure, may selectively be given a larger stability in standing, especially if lifting heights are large.
According to the invention, the object is attained by a fork-lift truck of the type described at the beginning the wheel bearing means of which has a shifting device interacting with the vehicle body, which selectively shifts the wheel bearing means with respect to the vehicle body in the vehicle longitudinal direction while varying the distance between the front and rear wheels and the distance between the centre of gravity of a load disposed on the load-carrying means and the front wheel. The fork-lift truck has a vehicle body on which the rear wheels are supported. The front wheels of the fork-lift truck are supported on the wheel bearing means which is connected to the vehicle body via the shifting device. At this point, each front wheel may have a wheel bearing means of its own or a common wheel bearing means for two front wheels may be provided. The shifting device causes the wheel bearing means and the vehicle body to be shifted relative to each other. The shifting action changes the distance between the front wheels and rear wheels of the fork-lift truck. Likewise, the distance between the load centre of a load disposed on the wheel bearing means and the front axle is changed in the opposite sense. Thus, the fork-lift truck has an axle base adjustable by the shifting device. The advantage in using a wheel bearing means with a shifting device is that the distance between the front and rear wheels can be adjusted by it. Since a tilting motion of a fork-lift truck handling a load is performed via the front axle shifting the front wheels away from the rear wheels of the vehicle body will displace the tilting edge to the front and reduce the tipping moment. At the same time, the righting moment of the vehicle body will be increased with the weight being the same.
In a particularly preferred aspect of the inventive fork-lift truck, the rear wheel has a braking device which, while responding to an actuation of the shifting device, blocks the rear wheel for the duration of the shifting operation. If the rear wheel is not blocked the front and rear wills will roll on the surface on which the fork-lift truck stands if the wheel position is varied. Depending on the nature of the surface on which the truck stands and the rolling drag on the wheels, the wheels will roll through a varying portion of the push length. For example, if the fork-lift truck has loaded a heavy load the rear wheel usually runs more easily so that if the wheel bearing means is shifted the vehicle body will be shifted rearwards by a certain portion of the shifting length. Such a rearward travel of the vehicle body is undesirable because the rearward travel makes it more difficult to manoeuvre and to precisely lift the load which was picked up. To avoid shifting backwards the vehicle body while the wheel bearing means is shifted forwards the rear wheel is blocked for the duration of the shifting operation whereas the front wheel is free to rotate.
As an alternative or in addition to the above described braking device, the fork-lift truck may have a front wheel provided with a drive and a control connected to a sensor where the sensor determines the push length traveled by the wheel bearing means with respect to the vehicle body and the control translates the push length determined into a control signal for the drive which causes the front wheel to roll in a sense of rotation and at an angle of rotation which correspond to the push length which has already been traveled. The purposive actuation of the front wheel prevents a counterforce to be applied to the vehicle body which will shift it rearwards. Rather, the front wheel is driven in order to move the wheel bearing means forwards, especially if the fork-lift truck carries a load. The driven front wheel rolling motion is such that the wheels are driven according to the push length. As a result of a controlled drive of the wheels, the position of the vehicle body is not changed by the shifting operation whereas the wheel bearing means moves away from the vehicle body or approaches it.
For a simple structure of the shifting device, it is opportune for the shifting device to have a linear guide element along which the wheel bearing means is designed to be shifted by a displacing element mounted on the vehicle body. Using a guide element permits to simply shift the wheel bearing means. Also, the two-part structure of the shifting device comprising a guide element and a displacing element proves to be easy in manufacture and particularly sturdy.
In a preferred embodiment of the inventive fork-lift truck, the guide element has at least one straight guide rail. Using a guide rail for the shifting device permits to simply incorporate the shifting device in the wheel bearing means and the vehicle body. The fact that two guide rails can be disposed in parallel and above each other in the lower area of the vehicle body makes it unnecessary to modify the rest of the vehicle body structure.
In a particularly advantageous embodiment of the inventive fork-lift truck, either side of the vehicle body has disposed on it a lower guide and an upper guide with the guides extending in parallel with each other. Using a lower guide and an upper guide increases the stability of the fork-lift truck, particularly if it picks up large loads.
In another useful aspect of the invention, the wheel bearing means provides two guide elements on it each of which is disposed laterally in the vicinity of the front wheels. Using two externally disposed guide elements increases the stability in connecting the wheel bearing means to the vehicle body. In particular, the arrangement of the guide elements which is comparatively far outside laterally permits to achieve a high anti-torsion stiffness between the wheel bearing means and the vehicle body. Also, guide elements thus disposed may be easily integrated in the vehicle body with no need to modify the arrangement of the components to a wide extent.
An advantageous embodiment of the inventive fork-lift truck includes hydraulic cylinders acting as displacing elements which are disposed each in parallel with the guide element. Such a parallel arrangement of a hydraulic cylinder saves space and ensures efficient force transmission. Also, it constitutes a simple option to transmit forces between the wheel bearing means and the vehicle body.
In a particularly advantageous aspect of the inventive fork-lift truck, the wheel bearing means is of a H-shaped form with the front wheels being substantially supported at the leg ends and the lift frame being disposed, in part, between the legs. As a result, the lift frame is closer to the front wheel axles, which reduces the tipping moment applied by a picked-up load to the vehicle body.
The lift frame advantageously is pivotally hinged to the vehicle body and has an inclining device which allows to adjust the position of the lift frame. Specifically, it proves opportune for the inclining device to act between the vehicle body and the lift frame.
The inventive fork-lift truck appropriately has a control device for an actuation of the inclining and shifting device, which actuation of the inclining device is effected according to the extent to which the wheel bearing means is shifted. This actuation enables the shiftable wheel bearing means to be flexibly used and, in addition, will increase the stability in standing by inclining the lift frame accordingly. The lift frame advantageously has sensors for the height and/or weight of a picked-up load. Depending on the data acquired which are transmitted to the control device the inclining and shifting device is actuated in dependence on the height and/or weight of the load. This permits to combine various stability-increasing measures depending on the height and/or weight. In particular, provisions can be made for the shift of the wheel bearing means, e.g. in case of a smaller load, to be effected less far than in case of a larger load.
According to the invention, the object is also attained by a fork-lift truck of the type mentioned at the beginning wherein the lift frame is pivotally hinged to the vehicle body and has an inclining device and wherein the wheel bearing means also is pivotally hinged to the vehicle body and has a pivoting device which pivots the wheel bearing means with respect to the vehicle body in the vehicle longitudinal direction while varying the distance between the front wheel and the rear wheel and the distance between the centre of gravity of a load disposed on the load-carrying means and the front wheel wherein a sensor is provided for determining the lifting height of the load-carrying means and is connected to a control device and the control device controls the actuation of the pivoting device in dependence on the lifting height in the sense of an increase in distance if the lifting height increases, wherein the inclining device, regardless of this, maintains the lift frame in a preset position or brings it into a preset position. In this embodiment of an inventive fork-lift truck, the wheel bearing means is pivotally hinged to the vehicle body. The pivoting motion of the wheel bearing means achieves the aforementioned advantages of an increased wheel base and a decreased tipping moment. In a fork-lift truck having a pivotable wheel bearing means, pivoting is effected in dependence on the lifting height of the load-carrying means and, hence, in dependence on the tipping moment. The positioning of the lift frame may be performed here regardless of the position of the pivoting device.
To prevent the vehicle body from moving backwards while the wheel bearing means is being pivoted the rear wheel may have a breaking device which, while responding to an actuation of the pivoting device, blocks the rear wheel for the duration of the pivoting operation. Like the shifting operation, the pivoting operation causes a backward motion of the vehicle body, which is undesirable. To prevent the vehicle body from supporting itself on the front wheels which rotate worse a provision is made that the rear wheels are blocked whilst the front wheels are free to rotate.
In a particularly preferred aspect of the inventive fork-lift truck, the front wheel, alternatively to or apart from having the above mentioned braking device, may have a drive and a control including a sensor wherein the sensor determines the pivoting distance traveled by the wheel bearing means with respect to the vehicle body and the control translates the pivoting distance determined into a control signal for the drive which causes the front wheel to roll in a sense of rotation and at an angle of rotation which correspond to the pivoting distance. Using the drive of the front wheel, the wheel bearing means travels in synchronism with the pivoting motion so that no counterforce is applied to the vehicle body.
In an appropriate aspect of the fork-lift truck, the inclining device acts between the wheel bearing means and the lift frame. In case of this hinge type of the inclining device, the lift frame swivels about the frame-mounted pivot with the point of engagement of the inclining device on the wheel bearing means moving relative to the vehicle body as the wheel bearing means is pivoted. The inclining device may be disposed such that an inclination of the lift frame may be performed both towards the vehicle body and away from it.
In order to maintain a lift frame position which is not dependent on the pivoting motion of the wheel bearing means the inclining device may actuate the lift frame in an opposed sense if the wheel bearing means is pivoted.
As an alternative to the last mentioned aspect, it may be useful that the inclining device acts between the vehicle body and the lift frame. With this attachment, the inclination of the lift frame is solely determined by the inclination device regardless of the position of the wheel bearing means. Expediently, a hydraulic cylinder is provided as a second displacing element.
In a preferred aspect of the inventive fork-lift truck, the wheel bearing means and the lift frame have a common pivoting axis. Such a common pivoting axis can be realized in a particular simple way by a hinge point which is common to the lift frame and the wheel bearing means. What further is advantageous in a common pivoting axis is that if a predetermined position is traveled to the swiveling motion is of a simple geometry.
In an appropriate aspect, the vehicle body mounts two parallel, forwardly projecting holder arms each of which pivotally supports a bearing arm for a front wheel. In this further aspect, the rear side of the lift frame includes bearing portions which are supported on the same axis as the bearing arms with the bearing arms being connected via a joining portion on which a displacing cylinder is hinged for the inclining device.